Laser position transducers play an increasingly important role in the kinematic and dynamic analysis of mechanisms An interference pattern generated between a reference light beam and a probe light beam reflecting off of a body of interest is used to derive positional information for the body of interest A basic assumption in all such laser position transducers is that the probe light beam is returned to an optical receiver despite a relative motion between the receiver and the probed body
Laser position transducers are known for determining a position of linear or rotary actuators. One type of system employs a laser output having two components of the same frequency. One component is sent directly to a receiver while the other component is reflected off a moving actuator before being sent to the receiver. The phase shift between the two components is converted into position information for the actuator. Another type of system employs a laser output having two different frequency components. One component is sent directly to a receiver while the other component is reflected off of a moving actuator before being sent to the receiver. The change in the beat frequency at the receiver due to actuator motion is converted into actuator position information.
For linear actuators, the laser beam can be simply reflected from a plane mirror rigidly attached to the actuator For rotary actuators, such as a disk drive actuator arm, corner cube reflectors have been employed to reflect the laser beam As is well known, a corner cube reflects a ray along a path parallel to an incident ray, independently of the incident angle.
However, relative to a plane mirror corner cubes are heavy, large and expensive. As such their suitability for certain applications, such as their inclusion within a servo-writing process for a magnetic recording device, results in several significant problems.
For example, a system for attaching and subsequently removing the corner cube from the drive's rotary actuator must be devised. As a result, the disk drive must typically be servo-written while not completely assembled. However, the presence of a relatively massive corner cube during servo-writing affects the natural frequencies of the disk drive. As a result, the subsequent removal of the corner cube and the completion of the assembly of a disk drive after it has been servo-written may cause distortions in the servo-pattern that results in track misregistration for the recording heads.
In U.S. Pat. No. 3,979,588, issued Sept. 7, 1976 (Park), in U.S Pat. No. 4,079,944, issued Mar. 21, 1978 (Durley, III et al.) and in U.S. Pat. No. 4,361,880, issued Nov. 30, 1982 (Kitamura et al) there are disclosed photoelectric sensors for use with phonograph records. Photoelectric sensors detect light reflecting from the smooth surface of the record between recorded portions. This technique provides a position sensor only to the extent that an on-target or off-target indication is indicated. This technique does not actually measure any change in position and, furthermore, there is no rotation involved.
In U.S. Pat. No. 4,118,039, Oct. 3, 1978, Kurata et al. employ a virtual image of a light source in a semi-transparent mirror as a pointer to show location on a phonograph record. This technique does not actually measure position nor does it involve rotation.
In commonly assigned U.S Pat. No. 4,558,313, issued Dec. 10, 1985, Garwin et al. disclose an indicator that relies on light reflecting from a rotating mirror and returning to a stationary receiver. The receiver is a photoelectric device that determines if the light ray has been blocked. This technique does not measure the position of a rotating body but instead uses an independently determined position of a rotating body to locate where a blockage occurs. Light is returned to the receiver by a surface that is made long enough to catch all scattered rays, the surface being covered with reflective beads The beads send the light back along paths approximately the same as the incident paths.
In U.S. Pat. No. 4,564,757, Jan. 14, 1986, LaBudde et al. disclose a position sensor for a pivoted galvo mirror in an optical disk read/write apparatus.
However, what is not taught in the prior art and what is thus an object of the invention is the provision of a simple and accurate technique, and apparatus for accomplishing same, for measuring a position of a rotary actuator. The technique employs a plane reflector attached to or integral with the actuator for twice reflecting laser light from a source to a corner cube and back to a receiver.